1. Field of the Invention
The present invention relates to an information recording/reproducing apparatus and method which can record, reproduce, and erase information along a track groove formed in a recording medium by utilizing physical phenomena caused when a probe is brought near a sample.
2. Related Background Art
A scanning tunneling microscope (to be abbreviated to an STM hereinafter) has recently been developed [G. Binnig et al., Helvetica Physica Acta, 55,726 (1982)], which allows direct observation of electron structures on and near the surface of a substance by utilizing physical phenomena (tunnel effect and the like) caused when a probe is brought near a sample. With this instrument, a real spatial image of a substance can be observed with high resolution regardless of whether the substance has a single-crystal or amorphous structure. The STM allows observation with low power without damaging a medium with a current. In addition, the STM can be operated in the open air and a solution as well as in an ultrahigh vacuum and can be applied to various materials. Therefore, STM is expected to find wide applications in the scientific fields and the fields of study and research.
In the industrial field, a great deal of attention has recently been paid to the principle of STM having a spatial resolution corresponding to an atomic or molecular size, and much effort has been made to realize a practical application of STM to an information recording/reproducing apparatus, using recording layers (e.g., thin films consisting of .pi.-electron organic compounds and chalcogen compounds) for recording media, as disclosed in Japanese Laid-Open Patent Application Nos. 63-161552 and 63-161553.
It is a principal object of such an information recording/reproducing apparatus to record information on the surface of a sample medium by some electrical method while sweeping the probe parallel to the sample surface, and reproduce the recorded information by measuring physical phenomena (a tunnel current or the like) caused when the probe is brought near the sample.
In such a case, in order to smoothly perform recording and reproduction of information, information must be aligned on the sample according to a certain rule. It is preferable that a groove for tracking (to be referred to as a track groove hereinafter) be formed in the recording medium, and that the track groove be detected and used as a reference to perform information write and read operations.
In an information recording/reproducing apparatus having the above-described arrangement, a tunnel current signal is used for detecting the track groove, similar to reproduction of recorded information. For example, at the instant that the probe passes a steep edge portion of the track groove, distance control cannot properly respond because the time constant of a distance control circuit for the sample and the probe is finite. As a result, the distance between the sample and the probe increases. At this time, a decrease in tunnel current is observed. The position of the groove can be detected by comparing the decrease in tunnel current with a reference current value.
Since a tunnel current signal is used for detecting the track groove, track groove detection is very sensitive to recesses/projections on the sample surface. For this reason, if the scanning speed is increased, the frequency of a correction output for a Z-direction control signal becomes higher than a mechanical resonance frequency in the Z direction. As a result, it becomes difficult for distance control between the sample and the probe to conform 100% to the spatial frequency of recesses/projections on the sample surface. That is, the relationship between the scanning speed and the sample-probe distance control becomes unstable, and the detected tunnel current is greatly influenced accordingly.
As is apparent, it is difficult to stably detect the track groove by using the above-described method, and hence the position of the track groove is erroneously detected.
Assume that an error occurs in the detection of the position of the track groove (actually, the detection of the position of the track groove edge), as described above. If a bit information signal is to be read by using the detected position as a reference, the possibility that information is read from a wrong portion increases, resulting in a deterioration in the reliability of the read information. For this reason, error correction must be performed to compensate for the deterioration in reliability, or the scanning speed needs to be decreased. This causes a decrease in recording information density due to an increase in the number of redundant bits, or a decrease in recording/reproducing speed.